Reinforced composite structure

ABSTRACT

A reinforced composite structure (29) is disclosed. The structure is formed by opposed layers of material extending over a core and continuous bundles stitched in a repeating pattern through the opposed layers (30, 32) and the intermediate core (28) to form the reinforced composite structural member (29).

This is a continuation-in-part of application Ser. No. 08/568,197 filedon Dec. 6, 1995 and entitled TANK FOR STORING PRESSURIZED GAS, now U.S.Pat. No. 5,647,503, which is a continuation-in-part of application Ser.No. 08/297,232 filed Aug. 29, 1994 and entitled NON-CYLINDRICAL FUELTANKS FOR NATURAL GAS VEHICLES, now abandoned.

FIELD OF THE INVENTION

The present invention relates generally to the construction ofreinforced composite structures, and more particularly, to a compositesandwich structure member.

BACKGROUND OF THE INVENTION

For many years, there has been interest in developing and usingalternative fuels for vehicles, and particularly, overland vehicles, forexample, automobiles, buses, trucks, etc. Over that period of time, manysuch vehicles have been retrofitted to operate using natural gas. Morerecently, with increasingly stringent air pollution standards, fleets ofvehicles that have been retrofitted to operate with natural gas are morecommon.

In currently retrofitted vehicles, the natural gas is often stored in acylindrically shaped pressurized metal vessel, such as, a steel oraluminum tank, designed specifically for storing gases such as naturalgas, propane, nitrogen, etc. under high pressure. The cylindrical shapeof the tank provides a circular cross section about an axis whicheliminates bending stresses and helps reduce the weight of the tank.Since the cylindrical steel natural gas storage tank is not suitable forand cannot be readily retrofitted in place of the vehicle's liquid fuelstorage tank, the natural gas storage tank is often housed in thestorage area or trunk of the vehicle, thereby eliminating or severelylimiting the use of the trunk for other storage. Therefore, there is aneed for a natural gas storage tank that can take the place of thevehicle's liquid fuel storage tank. Other gas storage tank designs andstructures are known in the art.

For example, the Pechstein U.S. Pat. No. 2,156,400 is directed to aspherical container for storing fluids such as gases and liquids. Thespherical container has a foundation with at least three reinforcingsupports adapted to transmit the forces exerted by the dead weight andthe weight of the contents of the container upon the foundation. Thecontainer further includes lower struts connected at their ends topoints on the inner wall of the container where the container rests onthe supports to form at least one lower polygonal frame. The containerfurther has upper struts connected at both ends to the inner wall of thecontainer at points lying in its horizontal middle portion to form atleast one upper polygonal frame. Inclined struts connect the cornerpoints of the upper and lower polygonal frames to provide a selfsupporting framework which is adapted to transfer the loads due to thedead weight and the weight of the contents of the container directlyupon the supports without substantially stressing the walls of thecontainer.

The Albrecht U.S. Pat. No. 2, 296,414 is directed to heavily reinforcedstorage tanks for liquids and gases that are present in high volume andhave angular sides made of flat or curved plates. The storage tank hasflat side, top and bottom walls of metal plates. A plurality ofvertically spaced tiers of braces are set at angles to adjacent verticalwalls. Each tier has a plurality of parallel, horizontal, equally spacedbraces lying in a common plane. Each of the braces forms a triangulartruss with adjacent vertical walls to cause the stresses in the bracingmembers and the wall plates to be compensating stresses.

The Pflederer U.S. Pat. No. 3,368,708 is directed to a filament woundstorage vessel capable of withstanding high internal pressures. Thecylindrical wall of the tank is formed of helically wound, fibrousmaterial impregnated with thermal setting resin serving to bond fiberstogether as an integral structure.

While all of the above known tanks are effective to confine a gas underhigh pressure, the designs of the tanks are directed to their particularapplication. For example, the design of the currently used steelcylindrical tank is directed to a tank that is intended to be portableand not permanently affixed to any particular structure. Therefore, thetank has specifications relating to its size, shape and weight thatfacilitate portability.

In contrast, the Pechstein '400 and Albrecht '414 patents are designedto store large volumes of pressurized gas and are not designed forportability. The Pflederer '708 patent is designed to have a removablehead portion at one end which presents different design considerationsand a different structure. None of the above tanks provide a tankstructure that may be constructed in any desired shape as may berequired for installation in a vehicle. Further, Applicants are notaware of any of those pressurized tank structures serving any purposeother than holding a pressurized liquid or gas.

SUMMARY OF THE INVENTION

The present invention provides a natural gas storage tank designedspecifically for installation in motor vehicles. Further, the naturalgas storage tank of the present invention has the capability of beingconstructed to any desired shape to fit the specifications and spacelimitations for installation in a motor vehicle. Further, it has beenfound that an intermediate structure created in the process offabricating the tank may be used as a composite sandwich structure.

More particularly, and in accordance with the principles of oneembodiment of the present invention, a fuel tank for a vehicle poweredby natural gas includes a three dimensional tank outer wall structuremade of a fibrous composite material. The tank outer wall structure hasan exterior surface and further has at least two walls bounding aninterior. The fuel tank further includes a set of continuous, fibrousbundles, for example, unidirectional, braided, twisted or monofilamentfibers, extending in a repeating pattern over the exterior surface ofthe outer wall structure on the first wall, through the first wall,through the interior, through the second wall, and over the exteriorsurface of the outer wall structure on the second wall.

In another aspect of the invention, the first and second walls of thetank may be parallel or may be adjacent, intersecting walls. In anotheraspect of the invention, the tank includes a second set of continuous,fibrous bundles extending in a repetitive pattern over the exteriorsurface of the wall structure on a third wall of the tank, through thethird wall, through the interior, through a fourth wall of the tank andover the exterior of the fourth wall.

In still another aspect of the invention, the tank includes a third setof continuous, fibrous bundles extending in a similar repeating patternover and through fifth and sixth walls of the tank. The first, secondand third bundles of continuous fibers may extend through the interiorof the tank in directions generally perpendicular to each other, or, indirections that are oblique to each other, or, in perpendicular andoblique combinations thereof. Therefore, advantageously, the walls ofthe tank may be adjacent.

The pressurized natural gas tank construction of the present inventionhas the advantage of being light in weight and capable of confining thepressurized gas. The construction permits the tank to be made in anygeometric shape and, preferably, in a noncylindrical, prismatic shapecomprised of a number of intersecting generally flat faces or surfaces.Therefore, the walls of the tank can conform to any available space in avehicle for a tank.

In accordance with another embodiment of the invention, a reinforcedcomposite sandwich structure includes opposed first and second layersthat extend over opposite sides of a core. A set of continuous, fibrousbundles extend in a repeating pattern over an exterior surface of thefirst layer, through the first layer, through the core, through thesecond layer and over an exterior surface of the second layer. Such astructure has a wide range of applications and the desired properties ofbeing very stiff and light weight. These and other objects andadvantages of the present invention will become more readily apparentduring the following detailed description together with the drawingsherein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a vehicle shown in phantom line andcontaining a the natural gas tank in accordance with the principles ofthe invention.

FIG. 2 is a fragmentary perspective view of the vehicle with the naturalgas tank of the present invention mounted in a different orientationwithin the vehicle.

FIG. 3 is a perspective view of the tank of FIG. 2 with parts phantomand parts in cross-section taken generally along the line 3--3 of FIG.2.

FIG. 4 is a fragmentary perspective view of a portion of the tank of thepresent invention with parts in cross-section illustrating anarrangement of fibers constituting a first fibrous network.

FIG. 5 is a fragmentary perspective view, similar to that of FIG. 4,with parts in cross-section illustrating two fibrous networks.

FIG. 6 is a fragmentary perspective view similar to those of FIGS. 4 and5 illustrating a three-dimensional fibrous network.

FIG. 7 is a diagrammatic view illustrating the force vectors whichoperate on the tank internally due to the pressures exerted by thenatural gas under high pressure.

FIG. 8 is a fragmentary, cross-sectional, perspective view of a portionof an embodiment of this invention illustrating the support of obliquewalls and the use of obliquely oriented fibrous networks.

FIG. 9 is a fragmentary, cross-sectional, perspective view of analternative embodiment of a reinforced composite structure in accordancewith the principles of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As will be apparent from FIGS. 1 and 2, the vehicle 10 contains highpressure tank gas tank 11 which, as shown, is located in the rear of thevehicle. In FIG. 1, the tank is oriented with its B axis in a verticaldirection, and its longitudinal C axis substantially perpendicular tothe length of the vehicle. In FIG. 2, the tank has been rotated aboutits longitudinal C axis so that its B axis in a horizontal direction.Attached to tank 11 is fill hose line 12 which is capable of handlingthe gaseous fuel under high pressure and engine fuel supply line 14which is also capable of handling the gaseous fuel under pressure.Pressure can be reduced at the tank fitting by use of a pressureregulator. Located to the rear of high pressure tank 11 and connected tofill hose 12 is a receptacle 13 for adding additional natural gas orother fuel.

As will be apparent from FIGS. 3--8, the outer structural wall portionof high pressure tank 11 is comprised of a material 15 which may be afibrous composite layered material, for example, a prepreg material, afilament wound toe, unidirectional or woven fabric which in turn, can bea fibrous mat, braided fabric or knitted fabric. The material 15 may bemade from unidirectionally or randomly oriented fibers.

As is noted from FIGS. 3, 4, 5, 6 and 8, continuous, internal, fibrousbundles 16, also referred to as "internal fibers", are arranged withintank 11 along the A axis as illustrated in the drawings. The fibrousbundles 16 can consist of monofilament line, wire or fiber that can beunidirectional, braided or twisted. More specifically, the fibers can bebundles of glass, quartz, graphite, organic and/or metallic fibers whichare joined together. Organic fibers that may be used include withoutlimitation aramid, nylon, polyethylene, and next generation organicfibers. Metallic fibers include without limitation steel and aluminum.The bundles of fibers may include either a single fiber or anycombination of fibers. The fibrous composite layers 15 and bundles 16may be coagulated together using a matrix material, which in turn can bea thermoset or thermoplastic resin or a metal matrix.

Exposed portions 17 of fibrous bundles 16 are crossed over or stitchedthrough the fibrous composite layered material 15 as shown in FIGS. 4,5, 6 and 8, and can be covered with a protective layer or coating 26.The protective layer or coating 26 can be, for example, a coating orprotective film such as rubber, urethane, vinyl, etc., or a thermoset orthermoplastic resin, a metal and/or a composite fibrous overlap.

The continuous, fibrous bundles 16 arranged along the A axis serve toprovide reinforcement substantially perpendicular to that of reinforcingfibrous bundles 18 and 20 shown in FIG. 6. The reinforcing fibrousbundles 16 pass through the multi-ply tank wall then exposed 17 alongthe exterior surface of the outer structural wall and then re-enter thetank through the wall. This pattern is repeated seriatim to provide theinternal reinforcement mainly along the A axis resisting the internalpressure forces, which would otherwise tend to warp the tank away fromits desired three-dimensional, noncylindrical structural configuration.The thickness of the wall 15 and the spacing of fibrous bundles 16 and17 can be varied as desired. Protective covering 26 can be a compositefibrous overwrap or layer or it can be a resinous coating. A liner 27can be used on the interior surface to meet the permeation requirementsfor specific applications. The liner 27 may be a urethane, silicon,isocyanate, or "TEFLON" material.

Continuous, fibrous bundles 18 are arranged mainly along the B axis asis shown in FIGS. 3, 5 and 6 and are crossed over/stitched through thewalls 15 and emanate along the outer structural wall as 19.

As shown from FIG. 6, disposed along the C axis are reinforcingcontinuous, fibrous bundles 20 whose exposed ends 21 emanate from theinterior of tank 11. These bundles pass through the multi-plies of theouter structural wall and are exposed on the exterior surface of theouter structural wall. The bundles then re-enter into the interior oftank 11 to serve as a lateral reinforcement, tying the multi-ply wallstogether so as to reinforce tank 11 from forces which would otherwisetend to push the tank out on all walls. The tanks of this invention arecharacterized as having a noncylindrical three-dimensional tank outerwall having an exterior surface and substantially opposed wall portionsof fibrous composite wall material and reinforcing portions. Thesereinforcing portions are in the form of a first set of continuous,fibrous bundles which traverse through the tank outer wall, and a secondset of continuous, fibrous bundles running in a direction substantiallyperpendicular to the first set and, also passing through the tank outerwall. The first set and the second set of fibrous bundles exit andre-enter the tank outer wall to provide exposed portions on the exteriorsurfaces thereof. A protective layer covers the exterior surface of thetank outer wall and these exposed portions of the fibrous bundles.

As will be apparent from FIG. 7, the vector forces operating internallyon the noncylindrical tank walls, which are of a multi-layeredconstruction shown at 11, 15, exert substantially perpendicularlyopposed forces. Forces from the fibrous bundles shown as 22, 23 balancethe gas pressure forces shown as 24, 25. As will be appreciated, fortanks of complex shapes, the forces shown as 22, 23 will not necessarilybe perpendicular. As will be apparent from FIG. 7, the respective pairsof forces 22, 24 and 23, 25 are in parallel but opposite directions.

The reinforcing fibrous bundles are depicted two dimensionally in axes Aand C in FIG. 8. One or more additional sets of reinforcing fibrousbundles can be located so as to be at reinforcing positions other thansubstantially perpendicular with respect to substantially opposed outerstructural wall portions. The fibrous reinforcing bundles can be placedat angles other than 90° to maintain complex shapes and/or to minimizethe number and length of the internal reinforced fibrous bundles,therefore maximizing tank volume. Fibrous bundles which are notsubstantially perpendicular to a wall surface are designed to balancethe forces such that the desired tank shape is maintained.

Such geometrically complex non-cylindrical fuel tanks in accordance withthis invention are characterized by a structure having fiber bundlesoriented in a crossing substantially perpendicularly intersectingpattern in combination with fiber bundles which are arranged at anglesother than substantially ninety degrees compared with the substantiallyperpendicular fiber bundles. Such structure is illustrated in FIG. 8 andcontains a complex geometric configuration having multiple plateausconnected by sloping spans and further characterized by rounded orsharply rounded edge surfaces.

In a lesser complex aspect, as shown in FIGS. 3, 5 and 6, thereinforcing structure illustrated involves plies of unidirectional tapeor woven fabric having a crossing 90° intersecting pattern involvingsubstantially perpendicular internal reinforcement. Thus the nature ofthe internal reinforcement and external reinforcement provided by thesebundles and woven or non-woven fabric containing them can be varied inaccordance with the present invention depending upon the specificpressure loads and the exterior wall engineering configuration of thetanks 11.

Fabrication

The tanks of the present invention can be made by a variety ofprocedures, including, but not necessarily limited to, procedureswherein the exterior tank wall is laid up, and in an enveloping fashion,covers a temporary or fugitive core through which the internal fibersare then stitched, or three dimensionally braided over a mandrel inwhich case a fugitive core is not needed.

The internal fibers or bundles of fibers as previously defined may bejoined together by a thermoset or a thermoplastic resinous matrixmaterial, or other matrix material. The metal fibers may joined by abrazing or soldering matrix material that is heated with the metalfibers at a temperature and for a time so that the metal fibers arejoined with the matrix material but do not become annealed. The matrixmaterial is capable of withstanding the solvents employed to remove thefoam or other temporary, viz., fugitive, core on the one hand or iscapable of withstanding the temperatures at which the foam or othertemporary core material is pyrolyzed once the internal and externalsubstantially perpendicular and non-perpendicular reinforcing fibershave been placed and solidified at their desired locations. Woven pliesof pre-impregnated material stitched with pre-impregnated bundles offibers can be formed by inflation followed by curing within assembledsections of a mold. Upon cooling or curing, the tank 11 achieves itssolid, non-cylindrical, three-dimensional desired configuration.

The liner material 27 may be added by two methods. The first methodinvolves placing the liner material, for example, a urethane, silicon,isocyanate, or "TEFLON" material, over the preform and under the fibrouscomposite material that forms the tank walls. The entire assembly isthen stitched with a set of fibrous bundles and then heat and/orpressure is applied to fuse the liner material to the assembly. Thesecond method involves filling the tank with a liquid, for example, aurethane, silicon, isocyanate, or "TEFLON" liquid, after the preform isdissolved; and then dumping the liquid out such that the internalsurfaces of the tank are completely coated with liquid. The internallyliquid-coated tank is then cured.

Additionally, with respect to the fabrication of non-cylindrical fueltanks 11 three-dimensional braiding techniques using a mandrel can beemployed without the use of core or fugitive materials on which toconstruct the tanks 11. Braiding techniques permit the tank 11 to retainits shape while resisting the internal pressure forces acting thereon,such is illustrated for example in FIG. 7.

One such technique for braiding without a core is the use of a braidedpre-form which has a thermoplastic resin previously incorporatedtherein. Such pre-resinified, pre-braided structures can then be heatedup and inflated to its final shape with a gas or liquid. The orientationand length of the fibers in the braided pre-form determine its ultimateshape.

Alternatively, a gas material can be injected into the interior of theresinified pre-form after it is placed within a female cavity of a mold,e.g., a mold formed from sections, so that the injected gas operates toforce the structure against the mold section into which ultimate shapetank 11 conforms. The heat can then be removed and the mold portionsseparated to result in the desired configuration.

The process used to attain functional rigidity of the tank is dependenton the matrix or resin material used. A thermosetting resin can be curedat room or elevated temperatures and a thermoplastic is final formed atelevated temperatures, then cooled.

The following nonlimiting example will further illustrate a storage tankconstructed in accordance with the principles of the present invention.

EXAMPLE

First, a core was formed from a piece of one inch thick foam cut into asix inch by six inch square. The edges of the foam were rounded, using aone-half inch router bit, thereby creating a foam square with smoothsemicircular edges having a one-half inch radius and two opposed fiveinch by five inch surfaces. Notches were cut at the center of twoopposed curved edges to receive metal inserts. The metal inserts weremade from a two inch long, one inch diameter piece of aluminum rod thatwas sawed in half longitudinally to create an insert with a semicircularcross-section. A longitudinal center hole was drilled through the metalinserts, and the holes were tapped to accept a 1/8 pipe. The metalinserts were then inserted into the notches so that their ends wereflush with the surface of the curved edge.

Next, three plies of three ounce per square yard E-glass woven fabricwere wrapped around the core, followed by three plies of twenty ounceE-glass woven fabric. The second set of plies of woven fabric wasrotated ninety degrees with respect to first set of plies. The fabriccovered foam core assembly was now ready to be stitched through thethickness. The stitches were made with a seventy pound tensile strengthbraided "KEVLAR" line in a grid pattern. The grid pattern had stitchingalong a first set of rows extending diagonally across the opposedsurfaces. Stitches also extended along a second set of diagonal rowssubstantially perpendicular to the first set of rows. The stitchespenetrated the fabric approximately every 0.125 inch, and the rows ofstitches were separated by approximately 0.125 inches, thereby tying thetwo five inch by five inch surfaces together. The fabric was cut fromaround the tapped holes in the metal inserts and two 1/8 pipes wereattached to the assembly. Epoxy resin was then squirted between thefabric and the foam, using a hypodermic needle. The resin was applied inthis fashion to ensure the resin thoroughly saturated the fabric. Thecompleted assembly was cured in one-half an hour and was allowed topost-cure for one week. Acetone was then used as a solvent to dissolvethe foam core, thereby forming the tank interior.

A T-fitting with two Zerk fittings was connected to one of the two 1/8pipes in the cured tank assembly. A 3,000 pounds per square inch ("psi")pressure gauge was attached to the opposite 1/8 pipe and was used torecord pressure. Testing began by first filling the tank with greasethrough the Zerk fittings. When the tank pressure reached 600 psi, asmall leak developed at one of the corners. The tank pressure was raisedto 1,000 psi, and it took twenty-nine seconds for the pressure to dropfrom 1,000 psi to 500 psi. Pressure was again applied to the tank, andthe internal fibers began to fail at approximately 1,300 psi. Byincreasing the number of stitches per square inch and/or by changing thetype of fabric or material, it is believed that a tank can be fabricatedin which an initial failure of the internal fibers will not occur untila pressure of 10,000 psi or more.

While manufacturing the tanks previously described, it was discoveredthat an intermediate structure provided significant alternative uses.More specifically, when the tank is manufactured around a captive core,prior to its removal, there is provided a very stiff, light weightreinforced composite structure. Referring to FIG. 9, the reinforcedcomposite structure 29 is formed by two opposed skins or layers 30, 32of fibrous composite material applied to a captive core 28. The core 28may be made from clay, foam, honeycomb, metal, paper, plastic, rubber,wax, or wood. The foam core may be metal, ceramic, rubber, concrete,plastic, or a polymer material. Further, the honeycomb core may be madefrom paper, composite, metal, plastic, a polymer, or thermoplasticmaterial. The layers 30,32 are substantially identical to the layer 15of FIG. 8 and are made of the same composite materials as previouslydescribed with respect to the layer 15.

Continuous internal fibrous bundles 34 are identical to the previouslydescribed internal fibrous bundles 16 of FIG. 8. However, in FIG. 9, thefibrous bundles 34 extend in a direction generally perpendicular to themajor surfaces of the layers 30, 32. Alternatively, the fibrous bundlesmay extend in a direction oblique to the major surfaces of the layers30, 32 as shown in phantom by the oblique bundles 36. The bundles offibers 34 are sewn or threaded in a repeating pattern over an exteriorsurface 38 of the layer 30, through the first layer 30, through the core28, through the second layer 32 and over the exterior surface 40 of thesecond layer 32.

A side 44 or end 46 of the reinforced composite structure 29 may be leftunfinished. Alternatively, the side and end surfaces may be finishedwith a close out or edge layer 48 that extends between the edges 50, 52of the respective layers 30, 32. The fibrous composite layers 30, 32 andbundles 34 may be coagulated together using a matrix material, which inturn can be a thermoset or thermoplastic resin or a metal matrix aspreviously described. Further, exposed portions 54 of fibrous bundles 34are crossed over or stitched through the fibrous composite layeredmaterial 30, 32 as described and can be covered with a protective layeror coating 56 that is identical to the protective coating 26 previouslydescribed.

The reinforced composite structure 29 is a sandwich construction thathas the capability of being made to any desired geometry or shape to fitany desired space. The reinforced composite structure 29 is especiallyuseful in the aircraft industry and can be used to make doors, panels,wings and structural members. In addition, the reinforced compositestructure can be used in an automobile chassis, bridges, floor membersfor buildings, trailers or trucks and grating, etc. The compositestructure 29 has the advantages of being very stiff, lightweight andcapable of possessing greater strength and stiffness when compared toother composite structures. In addition, the reinforced compositestructure 29 possesses increased damage tolerance and resistance todelamination.

While the invention has been set forth by a description of the preferredembodiment in considerable detail, it is not intended to restrict or inany way limit the claims to such detail. Additional advantages andmodifications readily appear to those who are skilled in the art. Forexample, the preferred embodiment of the invention is a noncylindricalfuel tank for storing pressurized natural gas for powering a vehicle. Aswill be appreciated, the construction of the present invention may beused in the construction of tanks of any geometric shape includingcylindrical tanks. Further, tanks constructed in accordance with thepresent invention may be used to store any gas under pressure, forexample, oxygen for aircraft emergency supply tanks, air for emergencyrescue and scuba tanks, nitrous oxide or other anesthetic in medicalenvironments, a propellant gas for a gun or weapon, propane in a lighterweight, more portable container. In addition, tanks constructed inaccordance with the present invention may be used for hydraulicaccumulators, fire extinguisher, tankard trucks, gas storage tanks forindustrial or commercial, etc.

The invention, therefore, in its broadest aspects, is not limited to thespecific detail shown and described. Consequently, departures may bemade from the details described herein without departing from the spiritand scope of the claims which follow.

What is claimed is:
 1. A stiff reinforced composite structurecomprising:a core; first and second layers of fibrous material, thefirst and second layers bounding respective first and second opposedsurfaces of the core and having respective exterior surfaces; and afirst set of continuous, fibrous bundles extending in a repeatingpatternover the exterior surface of the first layer, through the firstlayer, through the core; through the second layer, over the exteriorsurface of the second layer; and a matrix material joining the first andsecond layers of fibrous material, the fibrous bundles and the core intoa stiff reinforced composite structure.
 2. The reinforced compositestructure of claim 1 wherein the core is a material selected from thegroup consisting of clay, foam, a honeycomb, metal, paper, plasticsrubber, wax and wood.
 3. The reinforced composite structure of claim 1wherein the layers are a material selected from the group consisting ofa fibrous composite layered material, a woven fabric, a unidirectionallyoriented fiber arid a randomly oriented fiber.
 4. The reinforcedcomposite structure of claim 1 wherein the fibrous bundles are made froma material selected from the group consisting of glass fibers, quartzfibers, graphite fibers, organic fibers and steel fibers.
 5. Thereinforced composite structure of claim 4 wherein the steel fibers arejoined together by a lower melting point metal matrix.
 6. The reinforcedcomposite structure of claim 1 wherein the matrix material joins thefirst and second layers and portions of the fibrous bundles extendingover the exterior surfaces of the layers.
 7. The reinforced compositestructure of claim 6 wherein the matrix material is selected from thegroup consisting of a thermoset resinous matrix material and athermoplastic resinous matrix material.
 8. The reinforced compositestructure of claim 1 further comprising a protective layer coveringrespective exterior surfaces of the first and second layers and portionsof the fibrous bundles extending over the respective exterior surfaces.9. The reinforced composite structure of claim 8 wherein the protectivelayer is made from a material selected from the group consisting of aresinous coating, a protective film and a metal.
 10. The reinforcedcomposite structure of claim 1 further comprising an edge layerextending between edges of the first and second layers.
 11. Thereinforced composite structure of claim 1 wherein the fibrous bundlesextend between the first and second layers in a direction substantiallyperpendicular to the first and second layers.
 12. The reinforcedcomposite structure of claim 1 wherein the fibrous bundles extendbetween the first and second layers in a direction oblique to the firstand the second layers.
 13. A method of making a stiff reinforcedcomposite structure comprising the steps of:applying first and secondlayers of fibrous material over respective first and second opposedsurfaces of a core material; and sewing a set of continuous, fibrousbundles in a repeating patternover the exterior surface of the firstlayer, through the first layer, through the core material, through thesecond layer, over the exterior surface of the second layer, andcoagulating the first and second layers of fibrous material and thefibrous bundles with a matrix material; and curing the matrix material,thereby joining the fibrous material, the fibrous bundles, the core andthe matrix material into a stiff reinforced composite structure.
 14. Themethod of claim 13 wherein the method further comprises sewing thefibrous bundles in a generally perpendicular direction with respect tothe first and second layers.
 15. The method of claim 13 wherein themethod further comprises sewing the fibrous bundles in an obliquedirection with respect to the first and second layers.
 16. The method ofclaim 13 wherein the method further comprises applying an edge layerextending between edges of the first and second layers.